Mixtures of polycarbonates are known. See, for example, DT-OS (German Published Specification) No. 2,243,226, DT-OS (German Published Specification) No. 2,446,432, U.S. Pat. No. 3,038,874, U.S. Pat. No. 3,334,154, Canadian Patent Specification No. 725,726, U.S. Pat. No. 3,647,747, DT-OS (German Published Specification) No. 2,354,533 (LeA 15,318) and DT-OS (German Published Specification) No. 2,248,817 (LeA 14,668), as well as U.S. Pat. No. 3,166,606 and Canadian Patent Specification No. 702,625.
In each of the first seven literature references mentioned, one polycarbonate component of the polycarbonate mixtures is a halogen-containing polycarbonate.
In DT-OS (German Published Specification) No. 2,248,817 and U.S. Pat. No. 3,890,266, one polycarbonate component of the polycarbonate mixtures is a polycarbonate obtained from tetramethylbisphenols.
Polycarbonate mixtures with improved flow properties during extrusion and in injection molding are prepared according to U.S. Pat. No. 3,166,606 and Canadian Patent Specification No. 702,625. The polycarbonate components are characterized by their reduced viscosity. The high-molecular weight polycarbonate component has a reduced viscosity of at least 0.6 dl/g which corresponds to a weight average molecular weight, Mw, of at least about 32,000 for bisphenol-A-polycarbonates, and the low-molecular weight polycarbonate component has a reduced viscosity of not more than 0.5 dl/g which corresponds to a weight average molecular weight Mw, of not more than about 26,500 for bisphenol-A-polycarbonates. Moreover, the difference between the reduced viscosities of the polycarbonate components should be not less than 0.2 dl/g, and the reduced viscosity of the mixtures should be between 0.4 and 0.8 dl/g.
The polycarbonates obtained from bisphenol A which are usually suitable for injection moulding and extrusion are susceptible to stress cracking in the presence of certain organic liquids which do not dissolve polycarbonates and in the presence of unsaturated compounds, for example styrene solutions of unsaturated polyesters. Although increasing the molecular weight Mw of the polycarbonates to over about 70,000 indeed overcomes the tendency towards stress cracking, it is no solution to the problem because polycarbonates of this type with a Mw of over about 70,000 can no longer be processed on extruders to give films. The modification of polycarbonates by special branching according to DT-OS (German Published Specification) No. 2,254,917 (LeA 14,719) and U.S. Pat. No. 3,931,108 has thus been found as a way of producing extruded polycarbonate films which are resistant towards stress cracking.
The simpler way according to the present invention, namely of preparing polycarbonate mixtures which do not have the disadvantages of either one or the other polycarbonate component, by adding linear, high-molecular weight polycarbonates, which cannot be processed as thermoplastics without reduction in the molecular weight, to polycarbonates which are customarily suitable for injection molding and extrusion could not be predicted from the state of the art.